Composite materials are a useful class of materials that are often both very strong, and lightweight. Composites contain a mixture, or mechanical combination on a macro scale, of two or more materials that are solid in the finished state, are mutually insoluble, and differ in chemical nature. The use of composite materials in refractory applications is relatively new. Many states and forms of composite materials having desirable characteristics are currently being investigated.
In the field of composites, it is often desirable to incorporate ceramic-containing "whiskers" into polymeric, ceramic, etc. matrices. Whiskers are single, axially oriented, crystalline filaments of metals, refractory materials, carbon, boron, etc. that have high length to diameter ratios (aspect ratios). Ceramic whiskers have tensile strengths of from about 3 to about 6 million psi and have very high elastic moduli. Their useable upper temperature limit may be as high as 1700.degree. C., and in inert atmospheres, may be as high as 2500.degree. C. When ceramic whiskers are incorporated into the matrix of refractory materials such as ceramics, the overall strength and fractive toughness of the material can be increased. Such material properties are is important for many ceramics applications, e.g. cutting surfaces, internal combustion engines, turbine and propeller parts, spacecraft and airplane exteriors, etc.
Titanium nitride is one material that has been incorporated into composites. Titanium nitride is an attractive material because of its hardness, high temperature stability, electrical conductivity and other desirable mechanical properties. The physical state of titanium nitride can be in the form of titanium nitride whiskers to facilitate their incorporation into composites.
Titanium nitride whiskers (TiNw) have a relatively high melting point of about 2950.degree. C., and a hardness of 8 to 9 on the Mohs scale. TiN whiskers also exhibit good material strength, high temperature stability, and electrical conductance (for refractory materials). Titanium nitride whiskers have a tensile strength at least equal to silicon carbide whiskers (2000 to 6000 kpsi), and a higher Young's modulus and thermal expansion coefficient than SiC whiskers. Further, titanium nitride whiskers are thought to be inherently more compatible than SiC whiskers with other components in a composite. Thermo-chemical calculations indicate that TiN whiskers should be compatible, and stable in contact with SiC, Al.sub.2 O.sub.3, ZrO.sub.2, Si.sub.3 N.sub.4, and TiB.sub.2. Further, TiN whiskers have better chemical stability with iron aluminides and nickel aluminides than does SiC, and thus may find use in these and other metal matrix composites. Due to their higher thermal and electrical conductivity, TiN whiskers are also an excellent candidate for polymer matrix composite (PMC), ceramic matrix composite (CMC), and high temperature metal matrix composite (MMC) applications. For example, for applications requiring thermal management or electrical conductivity the compatibility of TiN whiskers with ferrous alloys, TiNw-containing Al.sub.2 O.sub.3 composites have been shown to perform well as cutting tools for ferrous alloys. The lack of available, economical TiN whiskers in commercial quantities has limited their widespread use.
Many approaches have been suggested in the literature for the successful synthesis of TiN powder, including vapor/plasma synthesis; thermal decomposition of titanium salts, sol-gel processing and chemical vapor deposition (CVD). However, the known processes for making TiN whiskers have exhibited low whiskers yields; inadequate for large scale manufacture. TiN whisker synthesis, methods, including carbothermal reduction-nitridation (CTN), thermal decomposition of salts, and chemical vapor deposition also have not produced satisfactory. In addition to low whisker yields, these processes require high reaction temperatures, furnace equipment, long reaction times of up to 72 hours, and are not otherwise conducive to large scale whisker manufacturing processes. These and other difficulties with TiN whisker-making methods have limited their feasibility and widespread use.